Optical communication signals can require signal regeneration over long optical fiber distances. In particular, with the advent of large-capacity optical fiber transmission based on Dense Wavelength Division Multiplexing (DWDM), optical amplifiers have become increasing important for the regeneration of DWDM signals carried in optical fibers. Erbium-doped fiber amplifiers (EDFAs) are particularly advantageous for optical signal regeneration because of their relatively low cost, high efficiency for amplification within the low-attenuation window of optical fibers, and low noise compared to other amplification schemes. In particular, erbium has useful gain at C-band wavelengths between about 1.53 μm and 1.62 μm where silica fibers have their lowest attenuation.
Conventional EDFAs comprise a short length of the optical fiber that is doped with a small amount of erbium added to the optical fiber glass as Er3+. Typically, the erbium ion is pumped by a high-power light beam (e.g., from a laser diode) that is mixed with a longer-wavelength optical signal. The excited erbium ions can give up energy in the form of photons by stimulated emission, thereby amplifying an optical signal having the proper phase and propagation direction.
The utility of EDFAs for amplifying DWDM signals is determined by the uniformity and width of the erbium gain profile. DWDM signals use many wavelengths, or channels, within the erbium gain profile. Erbium-doped fiber amplifiers can simultaneously amplify multiple weak light signals at wavelengths across their gain profile, enabling long-distance transmission without requiring separate repeaters for the various DWDM channels carried by the fiber. Unfortunately, the intrinsic gain profile of erbium is highly non-uniform. For short fibers, requiring few amplifiers, or for single-mode signals, this is not much of a problem. However, for long optical fibers requiring cascaded EDFA chains, the difference in amplification at the different DWDM wavelengths due to the non-uniform erbium gain profile can result in an imbalance in the optical signal power and the signal-to-noise ratio among the DWDM channels.
Channel spacing is an important consideration for both transmission capacity and performance. The narrowness of the intrinsic gain profile limits the usable bandwidth and, therefore, the channel spacing. To increase the data transmission rate, the communication industry has tried for years to squeeze more optical channels into the limited erbium bandwidth. As the number of channels is increased, the individual channel width must be decreased accordingly. This requirement, in turn, puts tremendous demand on the stability of laser emission wavelength, filter line-width, and standard of calibration. An enhancement of 2 to 3 times in usable bandwidth would relieve such a strict requirement. Conversely, within the same requirement, increasing the bandwidth could increase the number of available channels by 2 to 3 times.
A number of approaches have been proposed to flatten and broaden the gain profile of erbium and obtain uniform amplification and greater bandwidth of DWDM signals. For example, the gain may be controlled by the introduction of other dopants or the addition of gain equalizing spectral filters. However, these approaches can lower the efficiency or output power and may complicate system operation. Therefore, there remains a need for a means to obtain a flat erbium gain profile over a broad wavelength range without deterioration of the signal-to-noise or amplification efficiency.